The present invention relates to an LED light source with an LED as a light-emitting source used in an image reading apparatus as a light source, and an optical reduction type image reading apparatus using the LED light source as a light source.
Conventionally, as this kind of optical reduction type image reading apparatus, for example, such an apparatus has been known in Patent Document 1 [Japanese Laid-Open Patent Publication No. 2002-101263]. In FIG. 1, as shown in FIG. 1a, the optical reduction type image reading apparatus is comprised of a platen P for mounting a read original S, a first carriage body having a light source LA for lighting an original surface of the read original S placed on a mount surface of the platen P and a reflector M1 for reflecting the reflected light from the original surface of the read original S, a second carriage body having reflectors M2 and M3 for guiding the reflected light of the reflector M1 to a light-receiving means CCD, and as shown in FIG. 1b, a light-receiving section having an optical lens LE for condensing the reflected light from the reflector M3 of the second carriage body to form an image on a light-receiving surface of the light-receiving means CCD, and the light-receiving means CCD for receiving the image light of the read original S condensed by the optical lens LE.
Then, in this image reading apparatus, as shown in FIG. 1a, it is configured that the first carriage body is moved from a left corner to a right corner of the sheet along the original surface of the read original S at a velocity V, the second carriage body is moved in the same direction at a half velocity of the traveling velocity of the first carriage body while following movement of the first carriage body, and that the light-receiving means CCD reads the original surface of the read original S.
Meanwhile, as a light source LA used as a light-emitting source of the image reading apparatus, for example, an LED light source LA with an LED as a light source is known as described in Patent Document 2 [Japanese Laid-Open Patent Publication No. 2000-048616]. The LED light source LA is shown in FIG. 5. As shown in FIG. 5b, the light source is comprised of an LED that is a light-emitting source, and a light guide RG for guiding the light of the LED. Further, the light guide RG is provided with reflecting surface bodies RG1 protruding inside the light guide to retrieve the light emitted from the LED from the inside to the outside of the light guide so as to irradiate the original surface of the read original. As shown in FIG. 5c, the reflecting surface bodies RG1 are formed of dents in the shape of a V (or in the shape of an arc) at predetermined pitches p on the surface of the light guide RG.
First, the above-mentioned optical reduction type image reading apparatus has the following problem.
In other words, in the optical reduction type image reading apparatus, as shown in FIG. 2, even when the original surface of the read original S is irradiated with illumination light LA-Lv of the light source LA in the uniform light amount range LV1, the light amount of light received in the light-receiving means CCD through the optical lens LE undergoes the effect of cosine fourth law, and the light amounts LV2 and LV3 in the opposite end portions fall as shown in the figure. As a result, the center portion is bright, the opposite sides are dark, and unevenness develops in the light amount of the image as a whole. In addition, the cosine fourth law is that the image surface illuminance by incident light with any angle from the optical axis decreases in proportion to the fourth power of cosine of the incident angle (α) in relation to the image surface illuminance by incident light parallel with the optical axis.
Therefore, in the case of above-mentioned Patent Document 1, as shown in FIG. 3, the problem is solved by providing a slit plate SH with a slit aperture SH1 in between the optical lens LE and light-receiving means CCD. The slit aperture SH1 of the slit plate SH is opened so that the clearance is narrow in the center and broader as the clearance is closer to the opposite sides. Thus, as shown in FIG. 4, by reducing the amount of received light as approaching the center portion of the light-receiving CCD to uniform, unevenness in the light amount of the image is resolved.
However, this method of providing the slit plate SH is to reduce the entire light-amount level by cutting the amount of received light of the light-receiving means CCD, the whole of the image to read is darkened, and reading is sometimes made impossible due to a lack of the light amount.
Meanwhile, the following problem arises in the above-mentioned LED light source LA.
In other words, as shown in FIG. 5a, it is known that the LED light source LA provides the light-source characteristic LA-Lv with a phenomenon of peak occurring near the LED by concentration of light.
Therefore, in the case of above-mentioned Patent Document 2, the problem is solved as shown in FIG. 6b. In other words, the light source LED is separated from the light guide RG, an aperture plate Ep is provided in between the LED and RG, the light from the light source LED is thereby reduced, while the surface with the reflecting surface bodies RG1 formed is inclined a slant α so that the diameter of the light guide is narrowed as the distance from the LED increases, and as shown by the solid line in FIG. 6a, the phenomenon of peak is suppressed to almost uniform the light source characteristic LA-Lv.
However, in the method of gradually decreasing the diameter of the light guide so that the surface with the reflecting surface bodies RG1 formed is a slant α, there are limitations in the length of the light guide, and it is hard to prepare a light guide RG with a uniform light-source region. Further, when the light guide RG is formed by resin molding, the light guide becomes deformed by distortion, etc. due to flow and ambient temperature by varying the diameter of the light guide, and the deformation tends to be a factor of unevenness in the light amount.
The present invention was conceived under such circumstances, and it is an object of the invention to provide an LED light source which is optimal as a light source of image reading apparatuses, particularly enables unevenness in the light amount caused by the effect of cosine fourth law in the apparatus to be resolved on the light source side when the light source is used in an optical reduction type image reading apparatus, and is excellent in productivity.